Isothermal Oxidation Behavior of Nickel-based Superalloy U720Li in Different Oxidation Stages at 750 ℃

Abstract

The increasing service temperature of aero-engine turbine discs makes more stringent requirements for oxidation resistance of the materials. U720Li alloy is a high-strength nickel-based superalloy for turbine discs with a long-term service temperature up to 750 ℃. Clarifying the oxidation behavior at service temperatures of this alloy is of great significance for improving the service life of high-performance turbine discs. In this paper， the oxidation kinetics curve of forged U720Li alloy at 750 ℃ has been determined by the static weight gain method， and the oxide film morphology， structure and element distribution in different oxidation stages were analyzed by XRD and SEM-EDS. The results show that the U720Li alloy has excellent oxidation resistance at its service temperature， and its isothermal oxidation kinetic curve at 750 ℃ basically follows the cubic law， reaching the full anti-oxidant level. It has been analyzed that the loose γ-Cr2O3 is generated at the early oxidation stage of U720Li alloy. The γ-Cr2O3 layer spalls continuously as the oxidation proceeds， and the oxide film becomes more continuous and dense and forms a complex multilayer structure. Meanwhile， a γ´ depleted layer is gradually formed within the matrix nearby the oxide film. In the stable oxidation stage， the oxide film becomes more continuous and dense， forming a regular double-layer structure which includes the dense α-Cr2O3 outer layer and α-Al2O3 inner layer. The formation of continuous dense α-Cr2O3 and α-Al2O3 layers can effectively prevent the internal diffusion of oxygen and external diffusion of alloying elements， which guarantees the excellent oxidation resistance of U720Li alloy.

Keywords

• u720li alloy； oxidation behavior； oxide film； γ´ depleted layer； service temperature